Allergen Exposure

Geographical distributionExcept for a few feral populations, and one extremely endangered wild one (Przewalski's Horse), Horses are domesticated. Long used as a means of transportation, pleasure, work, and even war, Horses have been involved in much of human history. Domestic Horse breeds are numerous and highly various. Although little used for work today in developed countries, Horses are widely owned for recreational riding and show activities.

EnvironmentHorses are found in agricultural and recreational settings.

Unexpected exposureHorsehair is encountered in antique furniture, and Horse meat is eaten in some countries in place of Beef. Horsehide is used in making baseballs and certain other leather goods.

AllergensAt least 16 allergens have been isolated from Horse (1-4) . Several allergens have been shown to be glycoproteins, including a 27 and a 31 kDa protein (5).

A number of allergens have been characterised:

Equ c 1, Mw 25 kDa, a lipocalin (6-10).

Equ c 2, Mw 16 kDa, a lipocalin (6, 8, 10-12).

Equ c 3 Mw 67 kD albumin (Ag3) (10, 12).

Equ c 4, Mw 18.7 kDa (6, 12).

Equ c 5, Mw 16.7 kDa (6).

Various isoforms of Equ c 1 and Equ c 2, have been identified, including Equ c 2.0101 and Equ c 2.0102 (4, 11). Both allergens have been cloned from the sublingual salivary glands and have also been found to be expressed in the liver and submaxillary salivary glands (7).

The protein content of Horse dander is more than double that of Horse hair and skin scrapings, while the carbohydrate content is of the same order. The most important allergen in Horse dander appears to be a 27 kDa glycoprotein (the lipocalin Equ c 1). An important 67 kDa allergen was also isolated and was thought to probably be Horse albumin. Although common allergens appeared to be present in these extracts, unique allergens were present in all extracts (13).

Breeders and patients with asthma have claimed that Bashkir Horses are nonallergenic. A study demonstrated considerable inter-breed and within-breed variation but no breed-specific allergens. Dander from all breeds investigated contained the most important known allergens, and the allergenic content of dander from Bashkir Horses was similar to that of other breeds (14).

Horse allergens can be carried on clothing and can thus be found in domestic dust samples from urban environments (15).

Clinical Experience

IgE mediated reactionsHorse allergy occurs among people who regularly handle Horses, either professionally or for recreational purposes, resulting in the induction or exacerbation of asthma, allergic rhinitis, allergic conjunctivitis and occupational asthma (17-23). Horse allergy has also been reported to result in angioedema, respiratory distress, and poorly controlled asthma (15).

In a study reviewing children seen for allergy to Horses over a period of 11 years (35 boys and 21 girls, 35 of them were under 10 years of age), the main clinical signs reported were ocular symptoms (36), asthma (30) and rhinopharyngitis (24).

All the children had highly positive skin-specific IgE tests, and 62% had high specific IgE (class 3 and 4) and were polysensitised. In several children, the first symptoms occurred at the time of the first known contact with a Horse or Pony (24). Horse allergy has been reported to decrease with age (25).

A postal questionnaire sent to a random sample of 2,500 farmers throughout New Zealand reported that asthma prevalence was higher for Horse breeders/groomers (16.5%), Pig farmers (18.2%), poultry farmers (17.4%), and those working with Oats (17.4%). Hay fever was significantly higher in Deer and crop farmers, and farmers working with Horses and Goats; eczema was higher for Goat and Deer farmers (26).

The significance of Horse allergens in allergy has also been reported in Turkey (27), and in Indian subjects (28). In a Swedish study, Horse allergy was recorded in both boys and girls; it was noted that, whereas girls were more commonly horseback riders, boys were more often sensitised to Horses (29).

Skin-specific IgE responses to a variety of allergens, including Horse hair, has been reported to be higher in brittle compared to non-brittle asthma (30).

Other reactionsContact urticaria from Horse saliva has been reported (31).

A high proportion of workers on a pure-bred Horse farm showed a positive skin response to Saccharopolyspora rectivirgula (51.6%), or showed the presence of precipitins to Acinetobacter calcoaceticus (32.3%). No significant relationship could be found between the presence of symptoms and positive allergy reactions, and the possibility of Organic Dust Toxic Syndrome occurring in a high proportion of the workers was suggested (32).

Farmer's Lung is a rural disease that can be caused by inhalation of airborne Thermophilic actinomycetes. One case report is of an 11-year-old girl briefly exposed to this mold at a riding school (33).